1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
In order that the radiation that impinges on to the substrate is diffracted, objects with a specific profile are printed on to the substrate and are often known as scatterometry profiles. Ideally, the objects that are printed on to the substrate would have a predetermined shape and would be printed perfectly each time they were printed. However, because of the size (of the order of a few nanometers) of the objects, it is desirable to have a system to determine how exactly the objects are shaped; i.e. it is desirable to know the profiles of the objects. The objects may be diffraction gratings and the like which are made up of an array of bars or other periodic structures but have a cross-section, which is the profile from the surface of the substrate upwards.
As mentioned above, it is possible to determine the actual shape of a scatterometry object using scanning electron microscopes and the like. However, this involves a large amount of time, effort and apparatus.
Another way in which to determine the profile of a scatterometry object is to diffract a beam of radiation from the object and compare the diffraction pattern with model diffraction patterns that are stored in a library of diffraction patterns alongside the model profiles that create these model patterns. This general concept is known in the art. For example, U.S. Patent Application Publication 2003/0028358 A1 (Niu et al.) describes a system in which an actual signal from a scatterometry object is compared with a library of stored signals and the system tries to find a closest match of signals. The stored signals are each linked to an object profile parameter. An object profile parameter may be, for instance, the critical dimension (CD), a width of the object (which may vary with height), the height of the object or the angle of a side surface of the object, this angle being measured either from the surface of the substrate or from a normal to the substrate surface. The document goes on to describe the method as finding a closest match of a signal with each parameter of the scatterometry object. In other words, various possible parameters and possible permutations of parameters are tested to find a combination that gives rise to a signal that is as close to the actual signal that has come from the scatterometry object as possible. This gives a series of iterations of a “model signal”. This method is repeated iteratively until the model signal is as close as possible to the actual signal and then the model signal is stored alongside the parameters used. Finally, a computer checks a database comprising the parameters to determine if all parameter combinations have been entered. In a given example, for a simplified parameter set of three (a CD, a height and a width), if the range of the CD is 100 to 120 nm and the resolution is 1 nm, then there are 21 possible parameter values for CD. If there are also 21 possible values for height and 21 possible values for width, there are a total of 21×21×21=9261 possible parameter value combinations. The computer checks to see if all 9261 combinations have been simulated and stored in the database. The computer builds the database by simulating all of the possible combinations. Clearly, the problem with this system is that the greater the number of parameters, the greater the number of iterations that the computer must carry out and the greater the processing power and time that is required.
U.S. Patent Application Publication 2004/0210402 A1 (Opsal et al.) defines a system that aims to reduce the number of parameters required to build up the profile of an object from the scatterometry signals. The way the system does this is by providing “control points” around the outside of the profile shape from which the profile shape may be built up. For example, a square-profiled object has a single control point to show its height from the substrate surface and two points to show a width. The points are then joined up in a “dot-to-dot” fashion to give a line profile. The more complex the shape, the larger the number of control points is required to build up an accurate line profile. Furthermore, this system does not work well for overlapping shapes (e.g. complex shapes that look like overlapping simpler shapes or a profile that has a coating) or multiple shapes in a single profile, as the lines joining the dots may easily join the wrong dots.
Another problem with this system is that each control point will have at least one (if not two or three) degree of freedom. Each degree of freedom translates to a parameter that may change and the computing power is not reduced by very much, even though the parameters are changed.